L2J Server

For those of you who understand that the severe copyright issues in country which disregards them, such as china, doesn't really give a shit about GNU GPL, here's something I want to ask. I heared that there are people taking the source code of a project which is under GPL and modified them as their own software, I know they do have to right to charge on their modified work (which is granted by GPL), but are they require to release the source code to the public as well? If they are making money from it.

Sorry for my terrible english and logic, perhaps this sentence will make it clearier

Does the GPL require that source code of modified versions be posted to the public?

No, it doesn't. But if someone sells the binary files (compiled source) GPL says that it must be shipped together with the source code. It doesn't say that the source have to be public.
But, since the buyer receives all GPL rights, he can make it public ^^